Field of the Invention
The present invention relates to a method of making clad metal pipe by hot extrusion and, specifically, to a method of making clad metal pipe having a cladding layer of nickel alloy and a base of carbon steel, low alloy steel, or chrome-molybdenum steel.
Description of Related Art
Clad materials have been used widely in various applications. A clad material is a combination of two different types of metals or alloys that are adhered to one another such that the desirable characteristics of each of the metals can be utilized.
The clad material produced in the largest amount is clad steel plate in which one of the metals (called the “parent metal”) is carbon steel, low alloy steel, or the like, and the other cladding metal is stainless steel, titanium, or another corrosion resistant material. In this way, the corrosion resistance of the more expensive corrosion resistant material can be utilized on one or both sides of the less expensive parent metal to provide a lower cost product having the high corrosion resistance of the corrosion resistant material and the mechanical properties of the low cost parent metal.
Cladding has also been practiced in manufacturing many types of pipe. In the production of pipe, a pipe made of one metal is inserted into a pipe made from another metal to make a billet. The billet is heated to a high temperature, and then subjected to hot extrusion. Manufacturing costs and properties of the product pipe are important considerations in determining the materials to be used for the pipes. For example, for use in line piping used in the oil and gas industry, in which not only high strength but also improved resistance to corrosion are required, it is advantageous to use clad pipe comprising carbon steel or low alloy steel, which is less expensive and of high strength as the parent metal on the outside of the pipe, and a nickel-base alloy with improved resistance to corrosion as the cladding layer on the inside of the pipe.
A common nickel cladding material used in this process is described in U.S. Pat. No. 4,765,956. This nickel cladding material has a composition comprising: 6 to 12% molybdenum, 19 to 27% chromium, 2 to 5% niobium, up to 8% tungsten, up to 0.6% aluminum, up to 0.6% titanium, up to 0.03% carbon, up to 0.03% nitrogen, up to 0.35% silicon, the carbon, nitrogen, and silicon being correlated such that the sum of % carbon+% nitrogen+1/10% silicon is less than about 0.035%, up to 5% iron, and the balance nickel.
However, at any given temperature, the nickel based alloy has a higher resistance to deformation, i.e., higher high temperature strength, than the carbon steel or low alloy steel parent metal. This differential in resistance to deformation leads to delamination of the inner corrosion resistant alloy layer and/or surface defects in the cladding material including cracking and non-uniform thickness.
U.S. Pat. No. 5,056,209 to Ohashi et al. teaches that the resistance to deformation of both the parent metal and the nickel cladding tends to decrease as temperature increases and, thus, heating the billet to a high temperature before extrusion should help to reduce the defects in the nickel cladding. However, Ohashi points out that the heating temperature of the billet must be kept lower than the solidus temperature of the nickel cladding alloy so that intermetallic compounds concentrated along the grain boundaries do not turn into a liquid phase causing degradation in the ease of pipe formation and the properties of the product. Thus, Ohashi concludes that increasing the heating temperature of the billet is not a good way to solve the above-described problems. In addition, Ohashi states that it is impossible to completely remove the cracking only by heating the starting materials to a high temperature.
To solve the problem, Ohashi teaches heating the cladding material having the higher resistance to deformation to a higher temperature than the parent material prior to extrusion thus reducing the deformation resistance of the cladding material, bringing it closer to the deformation resistance of the parent material, and reducing the susceptibility of the cladding material to cracking.
However, the method of Ohashi requires additional processing steps and equipment making it costly and more difficult to perform. Also, the method of Ohashi has a fairly narrow temperature window for successful extrusion. Implementing such a narrow window makes practical extrusion procedures difficult to execute.